The search for ever more intriguing aromatic/antiaromatic species continues on – Haley has recently prepared the TIPS-protected indeno[1,2-b]fluorene 1. ¹ The crystal structure was determined and analogue with the tri-iso-propylsilyl groups replaced with hydrogens (2) has been computed at B3LYP/6-31G(d,p). This optimized structure is shown in Figure 1. The core system has 20 π-electrons – suggesting perhaps an antiaromatic system.

1

2

Figure 1. B3LYP/6-31G(d,p) optimized geometry of 2.

The x-ray structure and computed structure are in close agreement in terms of distances. The terminal phenyl rings exhibit very little alternation. The C₁-C₂ and C₂-C₃ distances are long (1.444 and 1.457 Å, respectively) while the C₁-C_3A and C­2-C₄ distances are short (1.379 and 1.396 Å, respectively.) This suggests a para-xylylene type structure for the central six-member ring. The NICS values of the terminal 6-member ring, the 5-member ring and the central 6-member ring are computed to be -7.12 (a reasonable phenyl value), 1.84, and 0.02. So the middle three rings possess no aromatic or antiaromatic character. Haley describes this structure as “a fully conjugated 20-π-electron hydrocarbon with fused s-trans 1,3-diene linkages across the top and bottom portions of the carbon skeleton”.

References

(1) Chase, D. T.; Rose, B. D.; McClintock, S. P.; Zakharov, L. N.; Haley, M. M., "Indeno[1,2-b]fluorenes: Fully Conjugated Antiaromatic Analogues of Acenes," Angew. Chem. Int. Ed., 2011, 50, 1127-1130, DOI: 10.1002/anie.201006312

InChIs

1: InChI=1/C64H92Si4/c1-41(2)65(42(3)4,43(5)6)37-33-57-53-29-25-27-31-55(53)61-60(36-40-68(50(19)20,51(21)22)52(23)24)64-58(34-38-66(44(7)8,45(9)10)46(11)12)54-30-26-28-32-56(54)62(64)59(63(57)61)35-39-67(47(13)14,48(15)16)49(17)18/h25-32,41-52H,1-24H3
InChIKey=JIXLLIXVMPJPEE-UHFFFAOYAI

2: InChI=1/C28H12/c1-5-17-21-13-9-11-15-23(21)27-20(8-4)26-18(6-2)22-14-10-12-16-24(22)28(26)19(7-3)25(17)27/h1-4,9-16H
InChIKey=DOQPHDKEGVECBF-UHFFFAOYAY

Proper handling of hydrogen bonding using DFT remains a concern. Sherrill has reported a careful benchmark study using the potential energy curves for the six dimer combinations involving formic acid, formamide, and formamidine.¹ Comparisons are made to the the CBS extrapolated limit CCSD(T) curve.

As anticipated, B3LYP and related functionals do a poor job. Interestingly, PBE and PBE0 provide very nice curves. The meta-GGA functionals like M05-2x and M06-2x and functionals with dispersion corrections (like ωB97X-D) provide very good potential energy curves. It is clear that intermediate and long-range correlation and dispersion must be accounted for when handling hydrogen bonded systems. Proper selection of the functional is critical.

References

(1) Thanthiriwatte, K. S.; Hohenstein, E. G.; Burns, L. A.; Sherrill, C. D., “Assessment of the Performance of DFT and DFT-D Methods for Describing Distance Dependence of Hydrogen-Bonded Interactions,” J. Chem. Theory Comput., 2011, 7, 88-96, DOI: 10.1021/ct100469b.